A key function of the microvascular endothelium is maintenance of barrier against fluid and solute transport. Breakdown of the retinal vasculature is a defining feature of some significant ocular diseases including diabetic retinopathy and age related macular degeneration. We propose to develop dual tracer fluorescence angiography as a novel quantitative tool for assessing retinal vascular permeability. The design of this Bioengineering Research Grant proposal identify specific features of this new technique for further development and testing. This dual tracer fluorescence angiography technique quantifies the permeability of the retinal vasculature by differential transport of small and large fluorescent tracers. We have implemented this retinal imaging technique in rats using the tracer pairs sodium fluorescein (376D) & Texas Red dextran (70kD) and, in fewer animals, using resorufin (235D) & FITC dextran (2,000kD). We have obtained preliminary induced by 5 minutes of mannitol infusion. Three aims for further studies are proposed. First, we plan to identify the best intravesicular and transvascular tracers for in vivo measurement of the permeability of the retinal microcirculation. Secondly, we propose to develop optical instrumentation and image analysis techniques for the simultaneous measurement of two fluorescent tracers in retinal vessels. And thirdly, we plan to develop mathematical models for the identification of retinal microcirculatory permeability characteristics from the dynamics of fluorescent tracers at the inlet and outlet of the retinal circulation. We will use simplified physical models, mathematical models, and principally, in vivo rat studies to carry out these aims. Our overall goal is to develop this methodology as a tool to measure retinal permeability which can be applied to diagnose and track the efficacy of treatments of this significant clinical retinal pathology which is a characteristic feature of many ocular diseases.